1. Field of Invention
The present invention relates generally to single-layer broadband reflective polarizers having electrically controllable polarization efficiencies and reflection bandwidths, and more particularly to single-layer reflective polarizers which can be electrically switched from broadband operation to narrow band operation over the visible and infra-red (IR) bands, as well as single-layer reflective polarizers which can be electrically switched from narrow band operation to broadband operation over the visible and IR bands.
2. Brief Description of the Prior Art
Humans have a natural affinity for pictures. Dramatic improvements in modern display devices are possible with the advent of novel polarizer technologies. In many advanced display techniques, a polarizing device having electrically controllable transmission characteristics such as polarization, transmittance, and spectra is desired in order to actively control the display brightness as well as color balance, for example.
Electrically controllable polarizers can also serve as an enabling technology for other applications such as smart window, wherein the window transmission is electrically switchable from a totally reflective state to a totally transparent state by incorporating two switchable polarizers with opposite polarization states. Such a window can provide lighting control and can additionally provide energy conservation benefits if externally mounted.
Since early attempts at utilizing cholesteric film as optical filters and polymer encapsulated nematic liquid crystals for display devices, there has been great attention focused on trying to bring polymeric liquid crystals and cholesteric liquid crystals together to make devices or light control application.
U.S. Pat. No. 5,691,789 discloses a single-layer reflective super broadband circular polarizer and method of fabricating the same by producing a single layer having cholesteric liquid crystal (CLC) order where the pitch of the liquid crystal order varies in a non linear fashion across the layer.
European Patent Application 0 643 121 A published Mar. 15, 1995 discloses a narrow band, switchable polarizing single layer reflector.
PCT application WO 97/2358, published Jul. 3, 1997, discloses a switchable polarizing single-layer reflector having a broader bandwidth.
General references on polymer dispersed liquid crystals may be found: xe2x80x9cPolymer Dispersed Liquid Crystal Displaysxe2x80x9d, by J. W. Doane; a chapter in xe2x80x9cLiquid Crystalsxe2x80x9d, Ed. B. Bahadur, World Scientific Publishing, Singapore; and xe2x80x9cCLC/Polymer Dispersion For Haze-Free Light Shuttersxe2x80x9d, by D. Yang et al. Appl. Phys. Lett. 60, 3102 (1992).
Smart Window Design is described in xe2x80x9cElectrochromism And Smart Window Designxe2x80x9d, by C. Granqvist, Solid State Ionics 53-56 (1992) and xe2x80x9cLarge Scale Electochromic Devices For Smart Windows And Absorbersxe2x80x9d, by T. Meisel and R. Baraun, SPIE 1728, 200 (1992).
The above identified U.S. patents and other prior art references are hereby incorporated by reference.
While there is a great need in the art for an improved single-layer electrically controllable broad-band reflective polarizer for use in diverse applications, prior art methods and technology have clearly failed to teach how to how to practice the same in a feasible manner.
Accordingly, a primary object of the present invention is to provide a single layer polarizing film having a very wide bandwidth which is switchable.
Another object of the invention is to provide a switchable reflecting polarizing filter having a very wide bandwidth which is controllable by an electric field.
Another object of the invention is to provide a switchable reflective film having little variation in the reflectivity outside of the reflective bandwidth of the film.
Another object of the invention is to provide a xe2x80x9csmart windowxe2x80x9d using a polarizing reflective film having a very wide bandwidth.
Another object of the invention is to provide a xe2x80x9csmart windowxe2x80x9d using a polarizing reflective film having a very wide bandwidth combined with a reflective multilayer polymer film having a very wide bandwidth.
Another object of the invention to provide a xe2x80x9csmart windowxe2x80x9d using a polarizing reflective film having a very wide bandwidth combined with a reflective multilayer polymer film having little variation in the reflectivity outside of the reflective bandwidth of the film.
Another object of the invention to provide a xe2x80x9csmart windowxe2x80x9d using a polarizing reflective multilayer polymer film having a very wide bandwidth combined with a light scattering layer for further control of transmitted light.
Another object of the invention to provide a reflective polarizing film having a bandwidth which is controllable by an electric field.
Another object of the invention to provide a xe2x80x9csmart windowxe2x80x9d using a polarizing reflective film having a very wide bandwidth which is controllable by an electric field.
Another object of the present invention is to provide an electrically-switchable family of infrared reflective polarizers and filters, based on the remarkable properties of cholesteric liquid crystals (CLCs), having far-reaching dual-use aerospace and window-glazing applications.
Another object of the present invention is to provide electrically controllable polarizers and filters that can be remotely controlled and involve no moving parts, to active-solar-control window glazings having the unheard-of property of infrared switchability while maintaining total visible transparency.
Another object of the present invention is to provide novel near-infrared switchable polarizers, filters, and reflectors, which fulfill a market need for remote-controlled, robust, thin-film, multiple-use optical components.
Another object of the present invention is to provide fast electrically-switchable infrared reflective polarizers capable of switching from broad-to-narrow band reflective operation over the IR band.
Another object of the present invention is to provide such a fast electrically-switchable infrared reflective polarizers, wherein its rise time is about 14.5 ms and its fall time about 8.5 ms.
Another object of the present invention is to provide fast electrically-switchable infrared reflective polarizers capable of switching from narrow-to-broad band reflective operation over the IR band.
Another object of the present invention is to provide a full understanding and comprehensive model of the chemical and physical switching mechanisms, verified through computer simulations.
Another object of the present invention is to provide an electrically-tunable infrared reflective polarizer.
Another object of the present invention is to provide an electrically switchable IR reflector based on an electrically switchable broadband reflective polarizer that operates in the IR region from 780 nm to 4 microns.
Another object of the present invention is to provide left- and right-handed CLC based switchable broadband polarizers that operate in the IR region from 780 nm to 4 microns.
Another object of the present invention is to provide a field-switchable broadband reflective polarizer operable in the NIR spectral region.
Another object of the present invention is to provide a field-switchable broadband reflective polarizer operable in the spectral region from 700 to  greater than 1000 nm, and having a polarizing bandwidth and extinction ratio which are changeable via an applied electric field. Another object of the present invention is to provide a novel method of optimizing the performance of such electro-optical structures in terms of extinction ratio, overall reflectivity, and reflection spectral cutting-off edge.
Another object of the present invention is to provide novel material recipes for making the switchable, broadband-to-narrow-band polarizers of the present invention, which enable further expansion of the polarizer bandwidth, shift to longer wavelengths, and increase the extinction ratio to the desired level.
Another object of the present invention is to provide a method of making such electrically-switchable IR reflective polarizers, using liquid crystal polymeric compounds having different pitch, cross-linking density, and polymerization rate.
Another object of the present invention is to provide an electrically-controllable narrow-band reflective polarizer which undergoes a shift in reflection band, rather than a broadening in bandwidth when a DC voltage is applied.
Another object of the present invention is to provide a novel method of precisely electrically tuning the CLC center wavelength by applying an electric field, without affecting the other specifications of the polarizer, such as polarization, extinction ratio, and bandwidth.
These and other objects of object will become apparent hereinafter and in the Claims to Invention appended hereto.
According to one of the broader aspects of the present invention, there is provided a single layer spectrum-controllable reflective circular polarizer having spectral characteristics which can be electrically controlled by application of an external electric field. The reflective polarizers are made from a cross-linkable cholesteric liquid crystal mixed with non-crosslinkable liquid crystal(s) and chiral dopant(s). These reflective polarizers reflect circularly polarized light matching its spiral sense.
In general, the present invention embraces two different types of single-layer spectrum-controllable reflective polarizer: a first-type spectrum-controllable reflective polarizer which switches from broadband reflective operation at a given polarization state, to narrow-band reflective operation at the given polarization state; and a second-type spectrum-controllable reflective polarizer which switches from narrow-band reflective operation at a given polarization state, to broadband reflective operation at the given polarization state.
The first type spectrum-controllable reflective polarizer, realizable in a 10 xcexcm configuration, can be switched from a broadband polarizing mode (having a reflection bandwidth from about 440 nm to about 660 nm) to a narrow band polarizing mode (having a reflection bandwidth from about 420 nm to about 460 nm) by applying an AC electric field.
The first-type of polarizer according to the present invention can be realized in the form of a single layer polarizing reflective film comprising a cross linked polymer matrix mixed with low molecular weight liquid crystal molecules. The liquid crystal molecules are oriented with respect to the surface of the film and to each other in cholesteric order, and the pitch of the cholesteric order varies non-linearly across the thickness of the film so that the film reflects circularly polarized light having a broad bandwidth when no external electric field is applied across the film. The ratio of the amount of NON-polymerization liquid crystal molecules to the amount of cross-linked polymer is chosen so that the liquid crystal molecules may rotate reversibly in an electric field, and hence destroy the cholesteric liquid crystalline order responsible for the broadband reflectivity of the polarized light. There is sufficient high molecular weight cross linked polymer material to ensure that the low molecular weight material does not diffuse after manufacture of the film, and to ensure hat the low molecular weight material returns to the cholesteric ordered state when the field is removed.
The second-type of reflective polarizer can be switched from a narrow band mode (having a reflection bandwidth from about 610 nm to about 680 nm) to a broadband mode (having a reflection bandwidth from about 480 nm to about 830 nm). The second-type of reflective polarizer according to the present invention can be realized in the form of a single layer polarizing reflective film comprising a crosslinked polymer matrix mixed with low molecular weight liquid crystal molecules. The low molecular weight liquid crystal molecules are oriented with respect to the surface of the film and to each other in cholesteric order. The ratio of the amount of liquid crystal molecules to the amount of cross-linked polymer is chosen so that the liquid crystal molecules may re-orient themselves in an electric field. The re-orientation of the low molecular weight molecules in the presence of an applied DC electric field perturbs the cholesteric liquid crystalline order responsible for the reflectivity of the polarized light. If the composition of the film is uniform, the polarized reflectivity of the film has a very narrow bandwidth when there is no electric field impressed in the film. As the electric field is increased, the bandwidth of the polarized reflectivity increases. There is sufficient high molecular weight cross linked polymer material to ensure that the low molecular weight material does not diffuse after manufacture of the film, and to ensure that the low molecular weight material returns to the original cholesteric liquid crystal ordered state when the field is removed.